1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to a zoom lens suited for a television camera, a video camera, a photographic camera, etc., and a photographing apparatus having the zoom lens.
2. Description of Related Art
Heretofore, in telephoto zoom lenses to be used outdoors, such as at a sports ground, among zoom lenses for television cameras or the like, there has been a demand for one that has the entire lens system reduced in size and that has a large aperture ratio, a high variable magnification ratio, and high optical performance. A zoom lens for the type in which focusing (adjustment of focus) is effected by moving a lens unit disposed on the object side of a variator lens unit has such characteristic features that, since zooming (variation of magnification) and focusing can be effected independent of each other, the mechanism for moving the lens units can be simplified, and, since the position of focus is not shifted due to zooming, focusing can be effected by a given amount of drawing-out movement with respect to a given object distance irrespective of the position of zooming.
Among such zoom lenses, in the so-called four-unit zoom lenses, each of which comprises, in order from the object side to the image side, a first lens unit (front lens unit) of positive refractive power for focusing, a second lens unit (variator lens unit) of negative refractive power for the variation of magnification, a third lens unit (compensator lens unit) of positive refractive power for compensating for the shift of an image plane due to the variation of magnification, an aperture shop, and a fourth lens unit (relay lens unit) of positive refractive power for image formation, the one that adopts the so-called inner focusing method, in which focusing is effected by moving a part of the first lens unit, has been disclosed in, for example, Japanese Laid-Open Pat. Application No. Hei 7-43611. In the four-unit zoom lens disclosed in Japanese Laid-Open Pat. Application No. Hei 7-43611, the first lens unit is divided into a plurality of lens subunits, and a lens subunit disposed closest to the object side in the plurality of lens subunits is made stationary during focusing, and a lens subunit disposed behind or on the image side of the lens subunit disposed closest to the object side is moved during focusing, so that the inner focusing method is adopted.
In general, a zoom lens of the inner focusing type has such characteristic features that the effective diameter of the first lens unit is smaller than that in a zoom lens in which focusing is effected by moving the whole first lens unit, so that it becomes easy to reduce the size of the entire lens system and it becomes easy to perform close-up photography, particularly, ultra-close-up photography, and, since focusing is effected by moving a relatively small and light lens subunit, a small driving force is sufficient for moving the lens subunit, so that rapid focusing is possible.
In order to make a zoom lens have a large aperture ratio (for example, the F-number being 1.8 to 3.3 or thereabout), a high variable magnifiction ratoi (for example, the variable magnification ration being 60x or more), and high optical performance over the entire variable magnification range and the entire focusing range, it is necessary to appropriately set the refractive power (optical power) of each lens unit, the lens construction, the sharing of achromatrism, etc.
In general, in order to obtain high optical performance with less variation of aberration over the entire variable magnification range and the entire focusing range, it becomes necessary to lessen the amount of aberration occuring at each lens unit by decreasing the power of each lens unit or increasing the degree of freedom for correction of aberration by increasing the number of constituent lens elements of each lens unit. Therefore, if it is intended to attain a zoom lens having a large aperture ratio and a high variable magnification, ratio, the air separation between the respective adjacent lens units becomes large inevitably or the number of lens elements increases inevitably, so that the problem arises that the entire lens system becomes heavy, thick, long and large. In addition, in order to obtain high optical performance over the entire zooming range and the entire focusing range in a high-definition broadcasting system, such as High-Vision, it is necessary to suppress the variation of aberration due to the zooming on the telephoto side or due to focusing. In particular, the variation of longitudinal chromatic aberration or lateral chromatic aberration or the absolute value itself of the amount of aberration must be suppressed as much as possible so as to obtain high resolution. At the same time, in such a telephoto zoom lens that the focal length at the telephoto end of the entire zoom lens system reaches sixty times the size of a photographic effective image plane, the manufacturing tolerance of the first lens unit (front lens unit) becomes strict, so that it becomes very difficult to attain the final optical performance as products. performance over the entire zooming range and the entire focusing range in a high-definition broadcasting system, such as High-Vision, it is necessary to suppress the variation of aberration due to the zooming on the telephoto side or due to the focusing. In particular, the variation of longitudinal chromatic aberration or lateral chromatic aberration or the absolute value itself of the amount of aberration must be suppressed as much as possible so as to obtain high resolution. At the same time, in such a telephoto zoom lens that the focal length at the telephoto end of the entire zoom lens system reaches sixty times the size of a photographic effective image plane, the manufacturing tolerance of the first lens unit (front lens unit) becomes strict, so that it becomes very difficult to attain the final optical performance as products.
In a zoom lens of the inner focusing type in which the first lens unit is divided into a plurality of lens subunits, and a lens subunit disposed on the most object side in the plurality of lens subunits is made stationary during focusing, and a lens subunit disposed behind or on the image side of the lens subunit disposed on the most object side is moved during focusing, as disclosed in the above-mentioned Japanese Laid-Open Pat. application No. Hei 7-43611, the stationary lens subunit is composed of at least one negative lens and at least one positive lens, and aberration occuring in the front lens unit is made to diverge by that negative lens. In such lens surfaces, as to the diverge aberration occuring in the front lens unit, particularly, a lens surface on the image side of the negative lens and a lens surface on the object side of the positive lens, the action of the diverging aberration occuring in the front lens unit is given very large weight. In such lens surfaces as to diverge aberration occurring in the front lens unit, particularly, a lens surface on the image side of the negative lens and a lens surface on the object side of the positive lens, the action of diverging aberration occurring in the front lens unit is given very large weight.
Therefore, in such a telephoto zoom lens that the focal length at the telephoto end of the entire zoom lens system reaches sixty times the size of a photographic effective image plane, when the radius of curvature of each of a lens surface on the image side of the negative lens and a lens surface on the object side of the positive lens becomes small with respect to the focal length at the telephoto end, the negative lens and the positive lens become the so-called effective lens whose manufacturing tolerance has a great influence on the final optical performance. Thus, there may be a possibility that the manufacturing tolerance in the radius of curvature, the lens thickness, the air separation, the refractive index, etc., of the stationary lens subunit of the front lens unit becomes strict, thereby making it difficult to manufacture such a stationary lens subunit.
In order to avoid the above possibility, it is necessary to cause the divergence of aberration occurring in the front lens unit to be shared by a plurality of portions without being concentrated on the stationary lens subunit only. Accordingly, if a concave lens is added to the focusing lens unit to diverge aberration, it becomes possible to decrease the amount of sharing of the stationary lens subunit for the correction of aberration, thereby greatly mitigating the manufacturing tolerance of the stationary lens subunit. However, in such a telephoto large-aperture zoom lens that the focal length at the telephoto end of the entire zoom lens system reaches sixty times the size of a photographic effective image plane, and the F-number at the telephoto end is 3.3 or thereabout, if a concave lens is added to the focusing lens unit to diverge aberration, the weight of the focusing lens unit increases remarkably, so that it becomes difficult to perform rapid focusing.
Therefore, particularly, in a telephoto zoom lens, the way in which the first lens unit (front lens unit) is composed, which greatly participates in the correction of aberration on the telephoto side or aberration due to focusing, becomes an important factor.
It is an object of the invention to provide a zoom lens having a large aperture ratio and a high variable magnification ratio and having high optical performance over the entire variable magnification range and the entire focusing range while adopting the four-unit zoom lens construction of the inner focusing type.
To attain the above object, in accordance with an aspect of the invention, there is provided a zooom lens, comprising, in order from an object side to an iamge side, a first lens unit of positive refractive power, a second lens unit of negative refractive power arranged to move during a variation of magnification, a third lens unit arranged to move during a variation of magnification, and a fourth lens unit of positive refractive power, wherein the first lens unit includes a first lens subunit arranged to be stationary during focusing, and wherein the first lens subunit includes, in order from the object side to the image side, at least one first positive lens, at least one negative lens of bi-concave form, and at least one second positive lens.
Further, in accordance with a preferred aspect of the invention, in the zoom lens, the following conditions are satisfied:
1.75 less than nd
60 less than xcexdp11f
90 less than xcexdp11r
where nd is a refractive index of the negative lens relative to d-line, and xcexdp11f and xcexdp11r are Abbe numbers of the first positive lens and the second positive lens, respectively.
Further, in accordance with a preferred aspect of the invention, in the zoom lens, the following condition is satisfied:
Pg,d less than 1.36xe2x88x920.00208xc3x97xcexdn11
provided that Pg,d=(ngxe2x88x92nd)/(nFxe2x88x92nC), where ng, nF and nC are refractive indices of the negative lens relative to g-line, F-line and C-line, respectively, and xcexdn11 is an Abbe number of the negative lens.
Further, in accordance with a preferred aspect of the invention, in the zoom lens, the following condition is satisfied:
60 less than xcexdp12
where xcexdp12 is a mean value of Abbe numbers of all positive lenses constituting the second lens subunit.
Further, in accordance with a preferred aspect of the invention, in the zoom lens, the second lens subunit consists of a plurality of positive lenses.
Further, in accordance with a preferred aspect of the invention, in the zoom lens, the following condition is satisfied:
70 less than xcexdp31
where xcexdp31 is an Abbe number of at least one positive lens among positive lenses constituting the third lens unit.
Further, in accordance with a preferred aspect of the invention, in the zoom lens, the image forming magnification of the second lens unit changes within a range including xe2x88x921xc3x97 during a variation of magnification, the third lens unit has a positive refractive power, the image forming magnification of the third lens unit changes within a range including xe2x88x921xc3x97 during variation of magnification, and the following conditions are satisfied:
55 less than fT/IS
1.0 less than FN1
provided that FN1=f1/(fT/FNT)
10 less than Z2
0.17 less than Z2/Z
where fT and FNT are the focal length and F-number at a telephoto end of the zoom lens, respectively, f1 is the focal length of the first lens unit, IS is the size of a photographic effective image plane, Z is a zoom ratio of the zoom lens, and Z2 is the rate of change of lateral magnification of the second lens unit.
The above and further objects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.